The present invention relates to a process for producing a diaphragm for a speaker of fully carbonaceous materials. More particularly, the invention relates to a process for producing a diaphragm for a speaker of fully carbonaceous materials having a high hardness, a high elasticity, a high strength, a light weight and a suitable internal loss as compared with conventional diaphragms, less deformation by external forces, small distortion of sound, wide reproducing sound range, distinct sound quality and adapted for a digital audio. The diaphragm is produced inexpensively by an industrially simple method. It is generally required that a diaphragm for a speaker satisfy the following conditions:
(1) large propagating velocity of sound,
(2) adequately large internal loss of vibration,
(3) large bending rigidity rate, and
(4) stability against variations in atmospheric conditions (no deformation or change of properties).
More specifically, the material for the diaphragm is required to reproduce in high-fidelity over a broad frequency band. To efficiently and distinctly produce sound quality, the material should have high rigidity, a light weight, and no distortion, such as creep, from external stress. The conventional materials used are paper, plastic and metals. However, while the paper and plastic have adequately large internal loss, they have a small propagating velocity of sound and are unstable to variations in atmospheric conditions. The metals have a larger propagating velocity than paper and plastic, but are still insufficient because of extremely small internal loss of vibration.
Recently, the use of a speaker diaphragm made of carbonaceous materials has been proposed due to the excellent features of carbon materials, i.e., a light weight, a high rigidity, an adequately large internal loss, and stability against variation in the atmosphere conditions such as temperature and moisture. More specifically, plastics to be carbonized or plastics in which carbon powder is dispersed are formed in sheets. These sheets are molded in a diaphragm shape by utilizing its deformation by heating, carbonizing and calcining.
The large propagating speed of sound is important among the features of the diaphragm. It is known that preferable results are obtained with a diaphragm of carbonaceous material due to a composite rule in a composite material of carbon fiber (particularly short fiber of carbon fiber having a high elastic rate) and crystalline graphite powder or graphite whisker as blended, as compared with material carbonized solely with an organic binder.
However, the carbon powder (especially the powder having a higher elastic rate) contains a crystal structure wherein carbon elements are regularly arranged with very small surface energy, thereby resulting in less affinity with the surface of an organic binder. Accordingly, the bond between the organic binder matrix and the carbon powder is weak when formed by merely dispersing, mixing and kneading the mixture of the organic binder and the carbon powder. Consequently, large bonding strength cannot exist in the boundary between the binder carbon carbonized from the organic binder and the carbon powder in the fine structure of the diaphragm obtained by the carbonization. Thus, the conventional diaphragm cannot satisfy the composite rule or utilize the excellent characteristics of the carbon powder due to the reasons described above.
Further, it is also known that, when a filler is used in the organic binder, the flexibility and the elongation of a preliminarily molded sheet decreases due to an increase in the viscosity of the mixed composition. The resulting product exhibits less moldability than the material in which no filler is used. Therefore, there is also the disadvantage that the quantity of the filler to be added is limited.